My husband's sons always had to contribute as much as they could toward school/cars, etc. and then my husband would pick up the difference. They never got "new" cars, always used. I don't know what the amount is he's asking you for but I'd ask him how much he has/is putting toward it and how long he's been saving for this thing he wants, etc. If it's just something that he basically wants and doesn't "need" (if the vehicle he's using now works) or if he's known about this "problem" for awhile, I'd let him work on saving up some money for most of the purchase. If he's not paying you rent, then his money is his own? and there's no excuse for him not to learn to save, etc.

Out of curiosity, who's going to pay for the tags/title/insurance, gas, etc.? He has to learn to be responsible at some point; start now?